Can I combine my GTX 980Ti with another card like GTX 1660?
You cannot run two graphics cards on the same tasks unless for Nvidia graphics cards unless they are the same make and you are using an SLI Bridge. You can have them on two seperate tasks if they are different but not the same. I have this motherboard but for Intel unfortunately, I did not know about thread rippers at the time. Could have had 6 more cores on top of ten for what I paid. But, this is the best I think. Works great on dual boot with CentOS. Just do not install gnome on it. Go with a Plasma KDE desktop. Gnome will most likely cause the login screen in CentOS to take along time to load if even at all. 

https://www.amazon.com/GIGABYTE-X399-Designare-EX-Motherboard/dp/B07662S766/ref=pd_sbs_147_9?_encoding=UTF8&amp;pd_rd_i=B07662S766&amp;pd_rd_r=540bdb43-7b92-4671-bb17-a62503bac9b1&amp;pd_rd_w=5KI67&amp;pd_rd_wg=yGvMq&amp;pf_rd_p=7c0dad87-8a25-4c4f-9349-026039ea6cb3&amp;pf_rd_r=2039ZHRGX36XF8JRM04J&amp;psc=1&amp;refRID=2039ZHRGX36XF8JRM04J